CERAMIC CAPACITORS

134

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Taping : dimensions

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II.2.2. Superconducting Detection Coil

This is a single piece of superconducting wire, configured as a second-order gradiome-

ter (see Fig. 3). This pick-up coil system is placed in the uniform magnetic field region

of the solenoidal superconducting magnet.

Fig. 3

II.2.3. SQUID

High sensitivity is possible because this device responds to a fraction of the flux quan-

tum. The SQUID device is usually a thin film that functions as an extremely sensitive

current-to-voltage-converter. A measurement is taken in this equipment by moving the

sample through the second-order gradiometer. Hence, the magnetic moment of the

sample induces an electric current in the pick-up coil system. A change in the magnetic

flux in these coils modifies the persistent current in the detection circuit. The current

change in the detection coils then produces a variation in the SQUID output voltage pro-

portional to the magnetic moment of the sample.

II.2.4. Superconducting Magnetic Shield

This is used to shield the SQUID sensor from the fluctuations of the ambient magnetic

field in the magnetometer’s location and from the large magnetic field produced by the

superconducting magnet.

II.2.5. Applications

This kind of equipment can be used to measure: (a) the real and imaginary components

of the AC magnetic susceptibility as a function of frequency, temperature, AC magnetic

field amplitude and DC magnetic field value; (b) the DC magnetic moment as a function

of temperature, DC magnetic field, and time.

II.3. Capsule Magnetization

The sample under testing has to be placed in a small capsule of 5-mm diameter and

8-mm length. Submitted to a magnetic field, the sample acquires a magnetization. The

capsule is then placed in a 6-cm tube which results in a field strength variation. This is

then measured in the SQUID and converted into a magnetization (uem-CGS unit in our

case). Thin paper is used to secure the sample in place inside the capsule.

Measurements were made with a controlled temperature (298.0±0.1K) and in the 0 to

3.5T range (35’000 Oe) as the magnetic field declined.

The first step before measuring any sample is to define the magnetization of the sam-

ple carrier, i.e. the capsule with some thin paper. Then, as the samples are measured, all

the magnetization values are corrected using the pattern below:

The signal from the capsule is diamagnetic and very weak. The signal from the capsule

+the paper assembly is more complex to determine, combining a strong diamagnetic

signal and a small ferromagnetic contribution (impurities in the paper material).

A constant corrective factor was then applied on all the measurements as a first ap-

proximation.

III. LABORATORY MEASUREMENTS

The aim of this study is to define a range of magnetization values within which electron-

ic components may be considered as non-magnetic and suitable for critical medical and

high RF power applications.

Several components were therefore tested to define a spectrum as wide as possible.

For instance, if we consider the high-Q multilayer ceramic capacitor: we started with

the chip alone, without even its terminations, adding a new variable - such as copper

or silver-palladium or nickel terminations, silver ribbons and finally laser marking - at

each subsequent stage. Using this protocol, it is easy to see the effect of each variable

on the final magnetization.

The magnetization in the charts below is given per gram. Each sample - or set of sam-

ples - is then weighed before the test run. The following designs were tested:

Designation

Number of Samples

Weight (mg)

Batch Number

501 CHB 4R7

3

155.1

C706527

501 CHB 4R7 BC

1

61.5

C649212

501 CHB 4R7 BC1L

1

133.6

C649212-0

501 CHB 4R7 BAL

1

57.5

52196

501 CHB 4R7 BS

1

61.2

C645208-2

silver leads type 1

2

55.4

CK/6297

silver leads type 2

2

57.2

CK/BC/2205

AT9401

1

58.6

OT0041006P

AT9402

1

60.8

OT0111206P

AT9410

2

194.0

OT0020806P

The descriptions of the samples used are as follows:

501 CHB 4R7

B size (1111) capacitor, 4.7pF, no termination

501 CHB 4R7 BC

B size (1111) capacitor, 4.7pF, copper termination

501 CHB 4R7 BC1L B size (1111) capacitor, 4.7pF, copper termination, leads

501 CHB 4R7 BAL B size (1111) capacitor, 4.7pF, silver-palladium termination

501 CHB 4R7 BS

B size (1111) capacitor, 4.7pF, nickel termination

silver leads type 1 silver leads used with B size capacitors

silver leads type 2 silver leads currently undergoing qualification

AT940

ceramic trimmer capacitor, 0.6 to 2.0pF, gold termination

AT9402

ceramic trimmer capacitor, 1.0 to 5.0pF, gold termination

AT9410

ceramic trimmer capacitor, 4.0 to 18pF, gold termination

III.1. DUT: 501 CHB 4R7

This sample shows a slightly paramagnetic behavior. Its magnetic susceptibility Xg is

around 10–7 uem.CGS/g which is a very low value.

General Information